The December 26, 2004 earthquake was recorded at many Global Seismic Network (GSN) stations
multiple times, because:

• seismic waves leave the earthquake's epicenter traveling in both directions around the Earth
• seismic waves keep traveling around the Earth, they do not stop after they reach a full 360 degrees

Seismic waves arrive at the GSN stations in the following order:

• R1 - The surface waves that travel the shortest route around the Earth reach the GSN stations for the first time
• R2 - The surface waves traveling the long way around the Earth eventually make it to the same GSN stations for the first time
• R3 - Another signal is recorded by the waves that traveled the shortest route, as they travel one full loop around the Earth and continue past the original epicenter to the GSN stations again
• R4 - The waves traveling the long way complete one full rotation and reach the GSN stations again